Conventionally, adsorption of solutes on solids is measured in static systems (test tubes, vials, etc.) followed by spectrophotometric determination of solute concentration in the supernatant liquid. This type of procedure is specifically suited to studies of adsorption equilibria, determination of adsorption isotherms, and subsequent calculation of specific surface areas. Determination of adsorption rates in static systems, although possible, is a tedious and time consuming task.
Existing dynamic techniques for measurement of adsorption from solutions normally employ a fixed-bed of adsorbent sample contained in a column which is initially filled with solvent or a carrier liquid. An analytical solution is then passed through the column and the concentration of eluate is continuously measured, typically by a photometric detector. As used herein the term "analytical solution" refers to any fluid for which adsorption on the solid is to be measured. It includes gases and/or liquids.
Typically, existing procedures involve the use of two liquid-phase delivery circuits, one circuit for pure carrier liquid and the other for the analytical solution. In such systems, carrier liquid is first passed through a cell containing solid sample and then through a concentration detector to obtain a carrier liquid baseline, i.e., the recorded output signal for no solute (dissolved substance) present at the detector. As soon as the baseline is established for the carrier liquid, the flow is switched from carrier liquid to analytical solution which now passes through the adsorbent bed. The recorded detector response is a breakthrough curve representing the solute concentration change as seen at the detector throughout the time of adsorption. The rate of adsorption as a function of time must be corrected by making allowance for the adsorption on the components of the empty sample column. The correction is a similar trace resulting from repeating the entire procedure with an empty column.
In the conventional method, the analytical solution must displace the carrier liquid present in the column containing a sample of adsorbent in order to appear at the concentration detector. This displacement causes a change in concentration from zero (pure carrier liquid) to that of the analytical solution used. Consequently, as the carrier is being displaced by the analytical solution, adsorption occurs on sample particles from a liquid phase of gradually increasing solute concentration. The sharpness of this change in concentration will depend on how efficiently the carrier liquid is displaced. This effect will vary from one sample of adsorbent to another, subject to differing particle sizes, porosities of samples, void volume of the column, and other parameters affecting adsorptive and hydrodynamic characteristics of the sample bed, sample lines and other pathways from the sample to the detector. This displacement of carrier by analytical solution will occur differently in an empty sample column than in a column packed with solid particles, e.g., due to different internal volumes in each case. Resulting adsorption rate profiles will then bear some error attributable to this particular procedure. Another serious drawback of the conventional method is the necessity to frequently replace pure solvent by analytical solution and vice versa, in case of multiple measurements, e.g., when samples of different adsorbents must be measured consecutively.
Heretofore, it has been thought necessary to employ a solvent or carrier for a variety of reasons. Amongst those are the need to remove all gases from the system and to establish a baseline before analytical solution is used. Thus, the state of the art now normally used in measuring adsorption rates includes as an essential step flushing of the system with solvent carrier and detection of background rates. However, the problems inherent in this procedure are several fold. Normally, it is time consuming. It also provides for a lack of sharp and distinctive adsorption lines for the analytical solution. This is so because there is a gradual change from background solvent carrier to analytical solution and the operator is never quite sure where one ends and the other begins. This lack of a distinct sharp break line makes interpretation of results more difficult, in addition to being time consuming.
Nevertheless, as an analytical technique, adsorption characteristic measurements are useful in a variety of analytical applications including measurements of turbidity, light transmission, acidity, and the like. Some examples of practical applications for which they may be used include characterization of mineral and coal surfaces by adsorption of dyes, determination of surface areas, examining the accessibility of porous structures to dye molecules, and qualitative evaluation of active carbon, etc.
Accordingly, it can be seen that there is a real and continuing need for improvements in analytical techniques involving measurement of adsorption profiles of solids.
This invention has as a primary objective to provide a means and method for adequate contacting of adsorbent with analytical solution which avoids the need for use of carrier solvents and which allows for sharp, distinct adsorption profiles in dynamic systems.
Another objective of the present invention is to provide a system which accomplishes the above referred to primary objective in an economical and efficient manner.
A yet further objective of the present method and system is to develop a system that requires only one liquid phase delivery circuit.
An even further objective of the present invention is to develop a system which has no need for carrier solvent and thus avoids the need for time consuming rinsing of the system with pure solvent before and between measurements.
Another objective of the present invention is to provide an adsorbent system and/or cell which allow monitoring of adsorption rates as early as one second after start of the test.
An even further objective is to develop a dynamic system which provides adsorption profiles which can be more easily interpreted.
The method and manner of accomplishing each of the above objectives will be apparent from the detailed description of the invention which follows hereinafter.